Human T lymphocytes have been found to undergo proliferation in response to the binding of foreign antigen to the antigen-receptor complex (Ti/T3 (CD3) complex) on the surface of cells. The molecular variability of this receptor conveys an exquisite specificity to individual T cells in their response to antigen. Recently, it has become clear that a number of other human T cell antigens (T11 (CD2); Tp67 (CD5); Tp44 (CD28)) may play major roles in the proliferation of T cells. Several of these putative receptors have also been found to synergise with the Ti/T3 complex in the stimulation of T cells. These synergistic pathways of activation may be important in the selective expansion of functionally-discrete subsets of T cells in their regulation of the immune response. In this proposal, we describe the identification of a novel T cell antigen, designated 2H1, which is capable of stimulating peripheral T lymphocytes in conjunction with the phorbol ester PMA. More importantly, this novel 2H1 antigen is also capable of synergising with the T11 antigen in the triggering of T cells. Immunprecipitation analysis has revealed that the 2H1 antigen is comprised of at least one subunit at 80,000-Mr which by the addition of multiple N-linked oligosaccharides and other forms of post-translational processing (such as phosphorylation) is converted to two forms at 105,000-Mr and 140,000-Mr; both of which are expressed on the surface of cells. No evidence from precipitation or co-modulation experiments was obtained to indicate that the 2H1 antigen is associated with the T11 and/or Ti/T3 complex. In this proposal, we propose (1) to obtain further information on the structure of the 2H1 antigen (orientation in the membrane, number of subunits, kinase activity, phosphamine etc.); (2) to purify the 2H1 antigen on a large scale for the generation of antisera and amino acid sequence data; (3) the isolation and characterization of a cDNA clone encoding the 2H1 antigen, followed possibly by transfection studies. Such reagents should provide powerful tools in understanding the mechanism by which the 2H1 antigen can stimulate T cells and interact functionally with other T cell antigens. Furthermore, information on the structure of the 2H1 antigen, as well as the isolation of a cDNA clone should facilitate an analysis of the expression of the 2H1 antigen at different stages of T cell differentiation and in various lympho- proliferative disorders.